Semiconductor memory can broadly be categorized into two major families that are characterized by whether or not their data is retained when power to the memory is removed. Volatile memory, which loses its data when the power is removed, is commonly referred as random access memory (RAM). Non-volatile memory retains its data when the power to the device is removed. Microcontrollers and microprocessors typically require some form of non-volatile and/or volatile memory for their basic operation.
Flash memory has replaced many traditional non-volatile memory types, such as erasable/programmable read-only memory (EPROM). Flash memory, with its ability to be reprogrammed in the system, eliminated the time consuming and costly efforts of having to erase EPROMs externally with an ultraviolet (UV) light source. Flash memory is becoming more popular in data storage applications in addition to code storage applications.
Data-storage applications are numerous and include digital still camera technology, voice/audio recorders, portable computers, and removable storage media. For example, with digital still cameras, flash memory is used as digital film. For voice/audio recorders, flash memory can be used as a storage media to replace tape. For portable computers, traditional disk drives can be replaced with flash memory, especially for harsh environments. Other applications include embedded processors. Embedded processors are essentially computers that are in appliances such as toasters, microwaves, VCRs, and other electronic devices, and that can retain set-up information (e.g., device programming and the state of the machine or device, etc.) all in memory.
One goal in the design of semiconductor memory is to achieve a high-density structure. Currently, memory density is limited by the size and structure of the memory cell, which is the basic unit of charge storage in memory. Another goal for manufacturers of semiconductor devices and memory is to reduce the number and complexity of the steps used in fabrication.